The present invention relates generally to pulse doppler radar systems and specifically to an automatic gain control (AGC) correction circuit providing dynamic gain restoration based upon variable returns from clutter and targets for an airborne early warning radar.
With the rapid advance in the development of weapons delivery systems, the early detection of airborne targets is increasingly critical to a successful defense.
Most pulse doppler radars have some form of fast automatic gain control (FAGC) to control the amplitude of signals entering the doppler filters. This FAGC acts independently on each range gate. A large target can reduce the gain through the FAGC and reduce the amplitude of smaller targets at different frequencies and drop them below the detection threshold (this assumes that the reference for the threshold is derived from other range gates). This process can result in a loss of detectability on small targets in the presence of large targets.
A particular pulse doppler radar mechanization, uses a digital FAGC. This mechanization uses a technique known as BASR (block adaptive signal regulation) for the FAGC function. This technique evaluates all of the time samples in a range gate before they enter the FFT. If a sample exceeds the dynamic range of the FFT, all samples are divided by 2.sup.N (where N is the number of BASR shifts) until the largest sample is within the dynamic range. This result in attenuation of that range gate by 6, 12, 18, 24, etc. dB depending on N.
The task of preventing the loss of detectability of small targets in the presence of large targets have been alleviated, to some degree by the following U.S. patents:
U.S. Pat. No. 3,787,849 issued to Sletten et al on 22 Jan. 1974;
U.S. Pat. No. 3,870,997 issued to Diperry on 11 Mar. 1975;
U.S. Pat. No. 4,058,809 issued to Chudleigh on 15 Nov. 1977;
U.S. Pat. No. 4,213,129 issued to Kennedy et al on 15 July 1980; and
U.S. Pat. No. 4,377,811 issued to Mooney, Jr. et al on 22 Mar. 1983.
The patent of Sletten et al discloses an airborne radar with a digital moving target detector which reduces the masking effect of ground clutter. The received signals are summed in N groups with each group including samples corresponding to range bins. Included is the use of an FFT algorithm and moving range gates behind doppler filters.
Cludleigh, Jr. disloses a doppler radar which prevents false target indication without inhibiting the display of moving targets in the main beam at the same range but having different doppler frequencies. The gain of a wide band return signal amplifier is separately adjusted for each range bin on the basis of the sampled amplitude of the return signal from the preceding radar pulse. In the patented device the gain is digitally controlled.
The patents of Sletten et al and Chudleigh, Jr. are instructive in the use of atuomatic gain control circuits in airborne early warning radar systems and are hereby incorporated by reference. However both the above references are directed towards the prevention of generation of false targets by ground clutter or terrain echo returns and the embodiment of those inventions are unsuited for the present need.
Mooney, Jr. et al disclose a pulse doppler radar having ground moving target rejection capabilities which is also unsuited for the present need.
Kennedy et al and Diperry both disclose the use of automatic gain control correcion circuits in pulse doppler radar systems and are hereby incorporated by reference. However, Kennedy et al discuss the adverse effects of a slow AGC and Diperry discloses a radar counter-countermeasure system operating with AGC and are unsuited for the present need.
Other prior art systems have used somewhat similar techniques for equalizing gain in the radar system. The known techniques have equalized gain over frequency to correct for the frequency dependent attenuation characteristics of the clutter filter. This type of gain compensation function is constant over time.
In view of the foregoing discussion it is apparent that there currently exists the need for a method of providing dynamic correction based on variable returns from clutter and targets which decreases the chances of returns from large targets from obscuring the returns of smaller targets. The present invention is directed towards satisfying that need.